Mechanism of proton-coupled electron transfer for quinone (Q{sub B}) reduction in reaction centers of Rb sphaeroides
- Univ. of California, San Diego, CA (United States)
- Univ. of Manchester (United Kingdom)
The mechanism of the proton-coupled electron transfer reaction, Q{sub A}{sup -}Q{sub B}{sup -} + H{sup +} {yields} Q{sub A}(Q{sub B}H){sup -} (i.e. k{sup (2)}{sub AB}), was studied in reaction centers (RCs) from the photosynthetic bacterium Rb. sphaeroides by substituting quinones with different redox potentials into the Q{sub A} site. These substitutions change the driving force for electron transfer without affecting proton transfer rates or proton binding equilibria around the Q{sub B} site. The measured rate constants, k{sup (2)}{sub AB}, increased with increasing electron driving force (by a factor of 10 per 160 meV change in redox free energy). The proton-coupled electron transfer was modeled. The free energy dependencies of these possible mechanisms were predicted using Marcus theory and were compared to the observed dependence. The best agreement with the experimental data is given by a two-step mechanism in which fast reversible proton transfer is followed by rate limiting electron transfer. For this mechanism the observed free energy dependence for k{sup (2)}{sub AB} can be fitted using reasonable parameters of the Marcus theory. The free energy dependence predicted using a simple model for a concerted reaction also provides a reasonable fit to the data. 75 refs., 9 figs., 2 tabs.
- OSTI ID:
- 420820
- Journal Information:
- Journal of the American Chemical Society, Vol. 118, Issue 38; Other Information: PBD: 25 Sep 1996
- Country of Publication:
- United States
- Language:
- English
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